The present invention relates to improving the data storage capacity of a data storage device.
Data storage devices, such as hard disks, floppy disks, optical drives or tape drives, are used to store data and operating instructions for computers. A typical disk drive comprises a number of disks each having a media surface coated with storage media to store computer data. For example, a media surface can comprise magnetic media which stores data in the form of two distinct magnetization states (corresponding to 0 and 1 in digital data) in a plurality of tracks. A magnetic head is coupled or paired with the media surface. Each paired magnetic head and media surface couples to provide a unique data recording capability which depends upon the fly height of the magnetic head from the media surface, the quality/distribution of magnetic media on the media surface, and the magnetic properties of the magnetic head.
Conventional methods of recording data using the paired magnetic head and media surface are inefficient because they do not take into account the differences in data recording capabilities between one pair of magnetic head and media surface and another head/surface pair. Typically, a single error code level and a single storage capacity level are used to record data for all the paired magnetic heads and media surfaces. This results in inefficient data storage for those pairs of magnetic heads and media surfaces that can store more data. It also lowers the qualification yields of the disk drives simply because one or more pairs of magnetic heads and media surfaces do not record data at the qualifying error rate and capacity levels.
The need for disk drives having higher data storage capacities increases as computer programs become larger and more complex. The disk capacity of a disk drive is the sum of the surface capacities of the media surfaces. The surface capacity is the total number of data bits that can be stored on a media surface. Data storage capacity can be increased by using a larger number of disks or by increasing their surface area. However, a larger surface area results in longer time delays in positioning a magnetic head over a particular track or portion on the disk. Moreover, increasing either the size or the number of disks will increase the external dimensions or “form factor” of the disk drive, which is against prevailing trends to shrink drive size. Also, larger disks or more numerous disks increase the energy consumption of the disk drive, which is also undesirable. Thus, it is desirable to increase the data storage capacity of a device without increasing disk size or number.
One method of increasing the data storage capacity of a disk drive comprises increasing the areal density of the data stored on the media surfaces (bits/sq. in.—BPSI). Areal density is the track density which is the number of tracks per radial inch (TPI) that can be packed onto the media surface, multiplied by the linear density (BPI) which is the number of bits of data that can be stored per linear inch. In addition, lower levels of error correcting codes (ECC) which are used to detect and correct errors in the retrieved data by adding extra parity bits or a number of redundant bits for each logical block of data (byte) that is stored on the media surface also increase data storage because of the added redundancy bits or parity bits. Typically, the track density, linear density, and level of error code are set to predetermined levels to ensure that 90% or more of the paired magnetic heads and media surfaces record data with less than the specified error rate.
Another problem arises because conventional processes for qualifying disk drives scrap a disk drive when the measured disk capacity of the disk drive is less than a target disk capacity. Conventionally, each media storage surface is formatted to store the same amount of data as every other media surface. Thus, a media surface that has a low error rate is formatted to the same TPI, BPI, and ECC levels, as a media surface having a high error rate, even though it can store more data. However, by adopting a single TPI, BPI, and ECC level for every media surface, current processes fail to account for the differences in sensitivity and accuracy of the paired magnetic head and media surfaces, which results in less data storage and more wastage of space on each media surface. Also, this results in lower overall yields of disk drives because if even a few of the media surfaces do not meet their targeted capacity, the sum of the surface capacities of all the media surfaces will be less than the target disk capacity, causing the entire disk drive to fail qualification.
The yield of disk drives is further lowered when the disk drive does not meet the desired qualifying error rate levels. Manufacturers often specify an upper limit on the maximum error rate that can be allowed in recovering stored data from a disk drive for it to be acceptable. Typically, the disk drive is assembled and formatted, data is stored on the drive, and then the stored data is read to calculate the error rate of the drive. Disk drives in which each paired head/surface has an error rate lower than the maximum error rate pass qualification while the other drives fail. Thus, it is desirable to have a method of testing disk drives that does not require the entire disk drive to be discarded if a single paired head and surface fail to meet the desired error performance level.
Accordingly, it is desirable to have an apparatus and method of storing data on data storage media that maximizes its data storage capacity. It is further desirable to compensate for the storage inefficiencies of particular pairs of magnetic heads and media surfaces. It is also desirable to have a disk drive with increased capacity and reduced error rates. It is further desirable to increase the yields of disk drives obtained during their qualification.